Material to be ground wet or to be dispersed in agitator mills is normally conveyed by a feed pump from below into a preferably cylindrical and very narrow grinding container and after it passes through the grinding zone it is removed at the upper end of the grinding container. A charge of grinding elements, which generally consists of approximately spherical grinding elements of steel, glass, ceramic or similar materials, is retained in the grinding container by means of a separation device; e.g., a screen or an annular slit. The material which is sufficiently ground or dispersed generally flows out through a lateral opening above the separation device.
The desired degree of fineness or dispersion of the material subjected to grinding is obtained on the one hand through a suitable arrangement or shape of the agitator mill and, on the other hand, through the selection of a certain charge of grinding elements, wherein the grinding is adjusted by the amount and size of the grinding elements, and through the regulation or control of the time-of-stay of the ground material in the grinding container, for which purpose the conveying power of the feed pump can be variable; e.g. by means of an infinitely variable gearing.
With the object of possibly varying the charge of grinding elements and also the density of packing of the grinding elements in the grinding container during the operation, there has been proposed in the prior art an agitator mill of the type described in British Pat. No. 1,314,789, whose piston, which can be displaced for the purpose of varying the density of packing of the grinding elements, has a piston rod or operation spindle which projects from the cylinder at the end extending away from the grinding container and is guided with possible axial displacement at such an end. In an embodiment of that agitator mill which comprises the cylinder mounted on the lower face of the grinding container, the inlet of the material to be ground is arranged approximately at the level of the lower end of the agitator shaft laterally on the grinding container. In another embodiment comprising the cylinder mounted on the side wall of the grinding container in the vicinity of its bottom, the inlet of the material to be ground is arranged in the botton of the grinding container coaxially with the agitator shaft. The axis of the cylinder extends in both cases at a right angle to the axis of the inlet for the material to be ground. Accordingly, there is the danger that the charge of the grinding elements may plug up the inlet for the material to be ground, if it is forced by the piston from the cylinder into the grinding container after the agitator shaft starts rotating. Indeed, in the case of the agitator mill of the prior art one could conceive the arrangement of the cylinder and the inlet for the material to be ground side by side on the bottom of the grinding container; however, this is possible only in the case where the cylinder possesses a considerably smaller diameter than the grinding container, which is again associated with the disadvantage that a considerable portion of the charge of the grinding elements may not be able to drop into the cylinder. The possibility of arranging both the cylinder and also the inlet for the material to be ground side by side on the side wall of the container is opposed by the fact that the flow of the material subjected to grinding passing through the inlet of the material to be ground into the grinding container would necessarily carry along a considerable portion of the grinding elements contained in the cylinder from the cylinder into the grinding container, even before the agitator shaft starts running, which is also the case where the axes of the cylinder and the inlet for the material to be ground extend at right angles to each other.
Meanwhile, the above discussed agitator mill possesses in relation to previously known mills the substantial advantage that at least a portion of the charge of the grinding elements passes into the cylinder when the piston is retracted. The said portion of the charge is thus removed from the grinding container so that even in the case where the grinding container is completely filled with the material to be ground the agitator shaft can be set in rotation with a driving power that is not substantially above the level of normal operating power since owing to the reduction in the charge of grinding elements in the grinding container the initial break-away torque of the agitator shaft is low.